Radio systems are sometimes used in environments where two systems share the same spectrum assignment and channels. The primary and secondary systems may have different deployment scenarios and may have different services. Secondary systems, for example, may be permitted to use unused local channels in the primary system as long as they do not cause interference or loss of performance to the users of the primary system. However, the transmitters of the secondary system may cause interference to the receivers of the primary system if the secondary system transmitters are in close proximity to the primary system receivers, i.e. the secondary system signals transmitted by the secondary system transmitters are received at the primary system receivers at a power that is greater than or equal to the power of the primary system signals received from the primary system transmitters.
Even when channel assignments for the primary and secondary systems are mutually exclusive (i.e., no channel is simultaneously assigned to both systems), when using adjacent (or nearby adjacent) channels, out-of-channel emissions or the spurious response of the primary system receivers may still cause interference and blocking of the primary system receivers. Blocking of the primary system receivers prevents the sharing of the unused channels by another service except in cases where the secondary system can be physically separated by a large distance from the primary system receivers so that the power of the secondary system signals transmitted by the secondary system are negligible in comparison to the primary system signals received at the primary system receivers. As primary system receivers may generally be located anywhere, such a requirement is not generally practical. Potential interference from a secondary system also prevents the reuse of adjacent channels in many system deployments, as “guard” channels are left between occupied channels in order to prevent interference from adjacent channels. It has therefore proven impractical to date to generally permit unused channels of the primary system to be used by a secondary system.